Sidebar - A friend of mine is putting together her portfolio, right now, and it includes a lot of video and stop-motion animation.

The stuff she did even 5 years ago is entirely unusable, apparently. Audio quality. Video resolution. Graininess. Whatever.

The NSA (et al) have been collecting data and storing it in massive troves forever.

Traffic cameras have been running with no film or recording devices for decades.

We may have already hit your threshold of "...crime serious enough for someone to troll back through the footage...", and that threshold is only going to get shorter and shorter as more and more content is created.

And then ... what? When storage/assessment becomes more costly than the crime itself, what's the point in recording at all ...?

There is definitely some deep human cognitive science to be done here ...

Naah you pick a record threshold by policy. The streams are so tidy at this point that with motion detection you can pack months on very little space. I've allowed my surveillance station to fill up 1TB, no more, and with nine 720P or better cameras, being constantly triggered by headlights and stuff, I can still go back about eight weeks.

Thing you gotta keep in mind is if it was built in the past twenty years, and it was worth surveilling, there were tape decks. Their archives were thin. But when those tape decks went away, the RAID arrays came in and they're phat.

If I dedicated my 40TB home NAS to recording the cameras I got I could do 24-7 recording of 9 streams at 18 frames for a month before I recycled anything and a 40TB array is Amazon Prime territory.